There are thin film transistors that use an amorphous oxide semiconductor. Such an oxide semiconductor can be formed uniformly as a film over a large surface area by, for example, sputtering at room temperature. The oxide semiconductor is transmissive in the visible region. The oxide semiconductor can be formed on a plastic film substrate that has poor heat resistance. Thereby, a flexible and transparent thin film transistor can be obtained. High reliability is desirable for the thin film transistor that uses the oxide semiconductor.